In the illumination devices that are presently common for surgical microscopes, halogen lamps are usually used for specimen field illumination. On the one hand these have a limited lifetime until total failure, and on the other hand the lamp's brightness decreases drastically in the course of its service life as a result of filament material deposited onto the lamp body.
It is known from U.S. Pat. No. 4,657,013 to use a dosimeter for patient protection in order to prevent excessive irradiation of the patient. In order to protect against a total failure of the lamp (with unforeseeable consequences) during an operation, it is proposed in the Zeiss brochure “OPMI® VISU 200 for Ophthalmology,” document no. 30-251-d, publication annotation IV/98, April 1998, to use a lamp changer which switches over immediately to a replacement lamp upon failure of the first lamp.
Operating hour counters are used to track the lifetime of xenon lamps, which are also used in microscope illumination systems. These counters are intended to allow operating personnel to replace the lamp after a specific number of operating hours has elapsed, so that it does not fail unexpectedly.
The inventor has recognized that the known systems are disadvantageous in the following ways:                a) After the lamp has aged to a specific degree, the operator (surgeon) justifiably complains about a lack of brightness in the surgical field, even though all the supply systems (electronics, light guide, optical coupling into the microscope) are OK.        b) This can even proceed to the point that the entire microscope is labeled as “bad” and unusable.        c) Advisories in the operating instructions explaining this effect are in practice seldom noted, or often are also not kept in mind at the critical moment in a hectic surgical situation.        d) In order to compensate for the brightness loss, as a rule the voltage is simply increased by turning the potentiometer or the light controller. This can result in a brightness shock when the lamp is changed, since the new, unused lamp is substantially brighter than the old one at the same voltage setting. In ophthalmology, this can result in damage to the patient's eye.        e) Except in the case of the operating hour counter system, it is not possible for maintenance personnel to predict when a lamp needs to be replaced before it fails at an inconvenient time.        f) Operating hour counters are designed only for average lamp outputs, and do not indicate the actual need for lamp replacement.        